1. Field of the Invention
The present invention relates to a driving device for driving a self-luminous display element, such as an EL (electroluminescence) element or an organic EL element, and more particularly to a driving device for voltage driving a self-luminous display element.
2. Description of the Related Art
In recent years, as a self-luminous display element, an EL (electroluminescence) element and an organic EL element have been put into practical use. The organic EL element realizes high luminance light emission and high efficiency, is driven with a DC low voltage, and has a high speed response. Therefore it is ideal for use as a light emitting display. Such a self-luminous display element has the following features: its light emission luminance is proportional to current density, as shown in FIG. 6; and its V-I characteristics are changed with the elapse of time, as shown in FIGS. 7 and 8, that is, it is deteriorated with time so that a voltage necessary for a current of a constant value to flow is changed. As a driving method of the self-luminous display element, there are two kinds of methods, constant-current driving and constant-voltage driving.
As a conventional driving device for driving the self-luminous display element, there is known a circuit for performing constant-current driving. In the case where the self-luminous display element is divided into a plurality of segments or dots, this constant-current driving circuit is provided for the respective dots or segments on a one-to-one basis, and drives the respective segments or dots. According to the constant-current driving circuit, since a constant current is supplied to the self-luminous display element, even if the self-luminous display element is deteriorated and the V-I characteristics are changed, a supplied current is not changed nor is the luminance changed.
However, since this conventional constant-current driving circuit must be provided for each dot or segment on a one-to-one basis, there have been disadvantages in that the device becomes large or complicated, and the cost is increased. Particularly, in the case where the areas of the respective segments are different from each other, since different constant-current driving circuits designed to correspond to the respective areas must be prepared, there has been a disadvantage in that the device becomes further complicated and the cost is increased.
As a conventional self-luminous display element driving device for solving the foregoing disadvantages, there is known a circuit for performing voltage driving. With respect to this voltage driving circuit, only one circuit is provided for a plurality of dots or segments, and applies a fixed constant voltage to the plurality of dots or segments. According to this voltage driving circuit, since only one voltage driving circuit is to be provided for the plurality of dots or segments, the device can be made simple or small, and the cost can be reduced.
However, according to the foregoing conventional voltage driving circuit, since a fixed constant voltage is applied to a self-luminous display element, there has been a problem in that in the case where the V-I characteristics of the self-luminous display element is changed with the elapse of time, current flowing to the self-luminous display element is changed by this and the luminance of the self-luminous display element is changed, so that suitable luminance can not be obtained.